That's interesting.. how did that happen???? Did a flywheel stopper somehow crack the rear iron in that location when you put the impact on the flywheel nut? Thats really the only possibility that comes to mind. I have lots of irons but only 6 port FC irons and they have been street ported so port size will not match your other irons.
You will need to provide more info as there are different rear irons. S4 or S5 FC? 4 port or 6 port engine? Of course the rotor side seals and oil seals ride on the inside of that iron, so you will need to inspect for step wear on whatever iron you buy. Also inspect the coolant seals grooves.